1. Field of the Invention
The present invention relates to a speed changer mechanism for use in an electrically powered tool such as a drill.
2. Prior Art
Some electrically powered tools have a speed changer mechanism for rotating a tool such as a drill bit selectively at a high speed or a low speed. Typically, such a speed changer mechanism comprises a pair of high- and low-speed gears rotatably mounted on an output shaft and held in mesh with larger- and smaller-diameter gears, respectively, on a transmission shaft which is rotatable by a motor. Between the high- and low-speed gears that are axially spaced from each other, there is a dog clutch plate axially movably splined to the output shaft. The dog clutch plate can be axially moved by a shift knob which is manually rotated by the user of the tool. When a low-speed mode of operation is selected, the shift knob is rotated to axially move the dog clutch plate in one direction until teeth thereon fit into recesses defined in one side of the low-speed gear. Now, rotation of the transmission shaft is transmitted from the smaller-diameter gear thereon through the low-speed gear and the dog clutch plate to the output shaft which then rotates a tool coupled thereto. In a high-speed mode of operation, the dog clutch plate is axially moved in the opposite direction to fit its teeth into recesses defined in one side of the high-speed gear. The output shaft is then rotated by the transmission shaft through the larger-diameter gear, the high-speed gear, and the dog clutch plate.
The high- and low-speed gears are normally urged by respective springs so as to be pressed against a cylindrical member of the shift knob which engages a peripheral surface of the dog clutch plate. However, the high- and low-speed gears tend to undergo localized loads because their peripheral edge portions are pressed against the cylindrical member of the shift knob. Accordingly, the high- and low-speed gears are liable to generate noise during operation, and have a short service life due to localized wear thereof.
Another problem is that the high- and low-speed gears and the dog clutch plate are required to be large and heavy to transmit the torque from the transmission shaft to the output shaft.